1. Field of the Invention
The present invention relates to a field effect transistor, in particular, to a field effect transistor that is formed on a GaAs substrate and that has excellent fabrication capability such as uniformity and reproducibility at a manufacturing operation.
2. Description of the Related Art
In recent years, a heterojunction type field effect transistor (FET) using excellent mobility characteristics of electron gas formed at a selectively-doped heterojunction interface has become attractive as a fundamental device of each communication equipment used in a microwave band and a millimeter-wave band and a fundamental device of an ultra high speed digital integrated circuit. In the heterojunction type FET, for example an undoped GaAs layer and an n-type AlGaAs layer doped with donor impurities are formed on a semiinsulating GaAs substrate so as to provide a heterojunction. When electrons are transitted from the n-type AlGaAs layer (referred to as the electron supply layer) to the undoped GaAs layer (referred to the channel layer), electrons are confined in the vicinity of the heterojunction interface. The two-dimensional electrons function as a channel. When the concentration of the two-dimensional electrons is controlled with a voltage applied to the gate electrode, the conductance between the source and drain can be controlled.
In the heterojunction FET, with a double-recess structure, both high voltage operation and large current operation can be achieved at the same time as disclosed in Huang et al., "An AlGaAs/InGaAs Pseudomorphic High Electron Mobility Transistor (PHEMT) For X- and Ku-band Power Applications", 1991 IEEE MTT-S International Microwave Symposium Digest, pp 713-716. FIG. 5 is a sectional view showing a principal portion of such a heterojunction type FET. In FIG. 5, reference numeral 1 is a semiinsulating GaAs substrate. Reference numeral 2 is a buffer type semiconductor layer that is composed of GaAs or AlGaAs or heterojunction type thereof that is not intentionally doped with n-type impurities or p-type impurities or doped therewith at satisfactorily low concentration. Reference numeral 3 is an AlGaAs electron supply layer (lower layer) doped with n-type impurities entirely or locally in the direction of the thickness thereof. Reference numeral 4 is a channel layer composed of for example GaAs or InGaAs that is not intentionally doped with n-type impurities or p-type impurities or that is doped therewith at satisfactorily low concentration. Reference numeral 5 is an AlGaAs electron supply layer (upper layer) that is intentionally doped with n-type impurities entirely or locally in the direction of the thickness thereof. Reference numeral 6 is a GaAs contact lower layer that is doped with n-type impurities. Reference numeral 7 is a GaAs contact upper layer that is doped with n-type impurities at high concentration. Reference numeral 8 is a source electrode. Reference numeral 9 is a gate electrode. Reference numeral 10 is a drain electrode.
The characteristics of the heterojunction type field effect transistor with the conventional structure depend on the width (w1) of a wide recess opening portion formed by etching out the GaAs contact upper layer 7, the depth (d1) of the recess thereof, the width (w2) of a narrow recess opening portion formed by etching out the GaAs contact lower layer 6, the depth (d2) of the recess thereof, and the donor concentrations (n1 and n2) of the GaAs contact upper layer 7 and the GaAs contact lower layer 6.
In the heterojunction type semiconductor layer composed of a compound semiconductor layer containing arsenic (As) and another compound semiconductor layer containing phosphorus (P), when a selective etching property using different etching solutions is employed, a Schottky barrier type field effect transistor (MESFET) with small fluctuation of drain saturated current can be accomplished as disclosed by Shoji Ohmura, Japanese Patent Laid-Open Publication 4-199641.
In the conventional field effect transistor using the heretojunction of the n-type AlGaAs layer and GaAs layer (or InGaAs layer), since the doping concentration of n-type AlGaAs is limited by the presence of deep levels, the concentration of the two-dimensional electrons is low. In addition, the threshold voltage largely depends on the temperature characteristics of electron distribution probability in the deep level. Masahiko Takigawa, Japanese Patent Laid-Open Publication 60-86872 and Tatsuya Ohbori et. al, Japanese Patent Laid-Open Publication 63-228763 disclose that such problems can be solved by a field effect transistor that employs the heterojunction of an n-type InGaP layer and a GaAs layer or a field effect transistor that employs the heterojunction of an n-type InGaP layer and an InGaAs layer.
In the process for fabricating the conventional double-recess structure heterojunction type field effect transistor shown in FIG. 5, the width (w1) of the wide recess opening portion, the depth (d1) of the recess thereof, the width (w2) of the narrow recess opening portion, and the depth (d2) of the recess thereof should be precisely adjusted and processed for the manufacturing. The control of the widths (w1 and w2) of the recess opening portions can be improved to some extent by the use of highly accurate lithography technology. However, when AlGaAs/GaAs are used as semiconductor materials, it is very difficult to accurately control the depths (d1 and d2) of the recesses. When AlGaAs of which the composition of Al is 0.3 or less and GaAs (or InGaAs) are etched out with different etching solutions, there is no satisfactory difference between the etching speeds. Thus, to control the etching depths of the recesses, the etching time should precisely be controlled. However, deterioration, improper composition control, and improper temperature control of the etching solution, and change of wet state of the etching solutions on the wafer surface adversely affect the accurate control of the etching depths of the recesses. Thus, the nonuniform etching in the double-recess structure occurs largely, leading to the nonuniform distribution in the characteristics (such as transconductance and gate breakdown voltage) of individual transistors on the wafer surface. Consequently, in the conventional transistor structure composed of materials of AlGaAs/GaAs, the double-recess structure cannot be stably provided for large volume fabrication.